1. Field of the Invention
The present invention relates to an ion implanter, and more specifically, to an ion injecting apparatus having means for generating a magnetic field for trapping secondary electrons generated when an ion beam irradiates a sample table or a sample.
2. Description of the Background Art
A conventional ion implanter will be described in the following with reference to FIGS. 1, 2, 3A and 3B.
FIG. 1 shows a sectional structure near a sample table of an ion implanter of Varian Associates disclosed in, for example, "Nuclear Instrument and Method in Physics Research B37/38 (1989), p492-p496". Referring to the figure, a sample 2 is held on a sample table 1 formed of aluminum or the like. A secondary electron trapping electrode 3 is arranged opposing the sample 2, for trapping secondary electrons generated when an ion beam (the arrow I.sub.B shown in FIG. 1) irradiates the sample table 1 or the sample 2. Negative potential electrodes 4 and 5 are arranged adjacent to the secondary electron trapping electrode 3 for trapping electrons therearound, and a beam stopper 6 for controlling the ion beam flow to the sample 2 is arranged between the negative potential electrodes 4 and 5. Not only the sample table 1 but the secondary electron trapping electrode 3, the negative potential electrodes 4 and 5 and the beam stopper 6 are formed of conductive materials such as aluminum.
The operation of the conventional ion implanter structured as above will be described. When the beam stopper 6 is set to "open", the ion beam irradiates the sample 2 which has been conveyed and placed on the sample table 1. The ion implanter is used in, for example, a process for forming a MOS transistor shown in a schematic cross sectional view of FIG. 2. Referring to FIG. 2, when a p type field layer 11 or a channel 12 is to be formed, for example, boron ions B.sup.+ are injected. When a source 13 or a drain 14 is to be formed, phosphorus ions (P.sup.+) or arsenic ions (As.sup.+) are injected. In either case, ions having positive charges are implanted in to the sample 2, and therefore the sample is positively charged if there is an insulating material of high resistance such as a resist 15 or a silicon oxide film on the surface of the wafer. When ions are implanted, secondary electrons incidental to the ion implantation are generated from the sample table 1 or from the sample 2. In such a conventional ion implanter, most of the secondary electrons generated from the sample table 1 or the sample 2 are trapped by the secondary electron trapping electrode 3. Further, most of the secondary electrons in the direction of the sample 2 (toward the right in FIG. 1) recombine with ions around the sample 2, and therefore few secondary electrons reach the central portion of the sample 2. Consequently, the electrostatic charge on the surface of the sample 2 becomes high, which causes electrostatic discharge damage, which is the cause of device defects generated at the central portion of the sample 2.
In order to solve part of the problem of the above described conventional ion implanter, an ion implanter neutralizing the positive ions with negative electrons such as shown in FIGS. 3A and 3B has been proposed in Japanese Published Patent Application 1-232653. Referring to FIGS. 3A and 3B, in the ion injecting apparatus disclosed in this publication, an ion beam 21 of positive ions 21a such as B.sup.+, As.sup.+, P.sup.+ and or Sb.sup.+ together with electrons 22 trapped by the ion beam 21 are applied to the wafer 23 to carry out ion implantation. The characteristic of this apparatus lies in the provision of a magnetic field generating source formed of, for example, 6 bar magnets 26 on a holder 25 of a wafer disk 24. The magnetic field generating source forms magnetic lines of force 27 on the surface 23a of the wafer 23. These magnets 26 are embedded radially in the holder 25 with the N pole of each of the magnets facing each other approximately at the center of the holder 25, and the S poles positioned on the outer periphery of the holder 25.
Since the magnets 26 are provided in the above described manner, magnetic lines of force 27 extending from the center of the wafer 23 to the outer periphery thereof are formed on the wafer surface 23a. The positive ions 21a reaching the wafer 23 enter the wafer 23, and at this time, positive charges 28 remain on the surface 23a of the wafer. Meanwhile, the electrons 22a reaching the wafer 23 with the positive ion 21a move on the wafer surface 23a. However, since magnetic lines of force 27 are formed on the wafer surface 23a, the electrons 22a move in a direction crossing the magnetic lines of force 27 in a cycloid movement in right-handed or left-handed rotation corresponding to the electron velocity at the time they reach the wafer surface 23a. During this movement, the electrons 22a collide with the positive charges 28 and neutralize the positive charges 28.
Since the electrons 22a move in a cycloid movement on the wafer surface 23a, the possibility of collision of the electrons 22a with the positive charges 28 is increased, and the positive charges 28 remaining on the wafer surface 23a are neutralized.
In the ion implanter disclosed in the above mentioned Japanese Published Patent Application 1-232653, the possibility of collisions of the electrons and ions is increased compared with a conventional device without bar magnets shown in FIG. 1, so that the electrostatic charge is decreased.
However, when electrons are not supplied from an electron generating apparatus and the sample 2 is an insulating body or an insulating material covers most of the surface of the sample, the rate of generation of the secondary electrons is lower than 1. Consequently, the effect of decreasing the charges by neutralization of ions with electrons can hardly be expected.
Even if electrons are supplied, there are many magnetic lines of force near the center and at the inner side of the outer periphery of the sample wafer surface, but few magnetic line of force exist on the outside of the outer periphery, since the magnets 26 are in the holder 25, as shown in FIGS. 3A and 3B. Due to the magnetic lines of force 27, the electrons reaching the wafer surface 23a move in a cycloid movement around the magnetic lines of force 27, collide with the positive ions and neutralize the positive ions. However, since there are few magnetic lines of force 27 trapping electrons not reaching the wafer surface 23a and the secondary electrons generated in the holder 25 near the outer periphery of the wafer 23, these electrons can not be effectively used for neutralizing the positive ions.